This invention generally relates to monitoring the amount of one or several substances accumulated in a container where the monitoring is done by observing the streams entering and leaving the container and the observation means are limited. More specifically, the invention deals with the task of determining the expected lifetime of a filter standing between two streams of gas.
When ultra high purity (UHP) product gases are supplied to semiconductor manufacturers, each of the supplied bulk process gases is typically purified before use by the manufacturer. In addition, these purified process gases are monitored continuously for certain key impurities. Some purifiers can be regenerated and have dual beds, one, on-line, purifying the product and the other regenerating or in a standby mode.
Three types of purifiers are most commonly used to provide purified process gases for use in semiconductor device manufacturing. First, there are the re-generable purifiers. These typically contain nickel metal immobilized on an inert solid support. The nickel metal can chemically remove oxygen and carbon monoxide. In addition, either the nickel metal or the solid support can physically absorb moisture, hydrogen, and carbon dioxide. One bed purifies the process gas at ambient temperature. The second bed regenerates at a high temperature, with the introduction of a reducing gas to regenerate the nickel metal from the oxide.
The second type of purifier is a consumable resin based getter, such as that made by Nanochem. This type of purifier is specific for the removal of oxygen, moisture, and carbon dioxide. A useful end of life test exists for this type of purifier as the resin changes color as the active material is expended, and a simple colorimetric sensor can monitor purifier lifetime.
The third type of consumable purifier is the heated transition metal getter. The unit operates at elevated temperature (350-400° C.) and chemically destroys or physically sorbs a number of impurities including nitrogen in inert gas streams. The purifier has varying capacity depending upon the specific impurity being considered.
Recently, the trend has been to use transition metal getter based purifiers, which are available to handle typical flow rates of gases such as helium, argon, nitrogen and hydrogen. These purifiers have the advantages of simplicity of operation, and the ability to remove a somewhat wider spectrum of impurities in a variety of process gasses. However, the one drawback to this type of purifier is that the getter material is consumable and cannot be regenerated. When spent, the purifier bed must be replaced at a typical cost of $50,000 per bed.
Knowing when to replace the purifier beds is a critical concern. Waiting too long runs the risk of allowing impurities to break through into the process gas stream resulting in lost product and process downtime. Replacing the beds at too short an interval incurs additional cost in replacing beds prematurely.
The continuous monitoring systems will alarm in the event of an impurity breakthrough, but this only detects a problem after it occurs. The optimal solution is to predict impending purifier breakthrough in time to take the appropriate corrective action.
For consumable purifiers, this involves determining the optimal time for bed replacement. For re-generable purifiers, this would allow optimization of the time between bed regenerations, which minimizes the power consumption.
Several methods to determine purifier end-of-life are generally known. One is to use the bed life estimates based on flow rate and inlet impurity levels provided by purifier manufacturers.
Most purifier manufacturers characterize their getter materials in terms of both spectrum of impurities removed and capacity of the material to remove each impurity per unit weight of getter material. Based on these parameters, many purifier manufacturers guarantee a purifier lifetime of one or two years for a given customer flow rate and impurity inlet challenge. For re-generable getter materials these tests determine the time between bed regenerations.
Although flow rate through the purifier is typically a measured quantity, and its effect on purifier lifetime is linear, impurity inlet challenge is typically not measured. Lower than expected impurity levels can lead to premature replacement of purifier beds or unnecessary bed regenerations. Higher inlet challenges can shorten purifier life and cause degradation of the purified process gas before the purifier is considered “spent.” In other words, this method may overestimate or underestimate bed life depending on the changing inlet impurity challenge and customer product demand.
Another solution to the problem is to monitor the purifier effluent for a number of impurities and sound an alarm when one or more of the impurities break through into the purified process gas. This approach is attractive as most facilities have continuous monitoring in place for most process gases. The problem with this approach is that it is not proactive. An alarm is sounded only after an impurity breakthrough has occurred. One could consider setting the impurity alarms at a lower concentration than the manufacturing process can tolerate in order that a corrective action can be taken before these impurity concentrations increase to an intolerable level. However, the usefulness of this approach is limited, as any increase of impurities above the detection limits of the various analyzers is considered detrimental and unacceptable to the chip manufacturing process.
Yet another solution is to use a purifier end of life sensor alerting the user when the bed breakthrough is imminent. For example, transition metal based getter materials swell as they consume impurities. This causes changes in both the volume and the electrical characteristics of the getter materials. Several patents have been issued on various methods that take advantage of this effect.
As the material swells, the porosity of the purifier bed decreases, and the resistance to flow increases. Therefore, to maintain the flow rate, the differential pressure across the purifier also increases. The differential pressure rise can be measured with one or more pressure sensors and correlated to purifier lifetime, and breakthrough time as described in U.S. Pat. No. 5,150,604.
As the getter material swells, the resulting pressure expands the containment vessel slightly. This expansion can be measured with a strain gauge in lieu of the pressure sensors described above. Correlation of strain with purifier lifetime is similar to that described for the pressure sensors. This method is described in U.S. Pat. No. 5,151,251.
As an oxide layer forms on the getter material, the electrical resistance changes with time, which can also be correlated with purifier lifetime. Using this type of measurement is described in U.S. Pat. No. 5,172,066.
A pressure sensitive switch can be embedded in the getter material, and swelling of the material can compress the switch and trip an alarm to indicate the end of purifier life has been reached. This approach is described in U.S. Pat. No. 5,294,407.
All the sensor technologies depending upon volume or resistance change suffer from a common disadvantage. Not all impurities affect the bed material the same way, and, more importantly, the purifier material has differing capacities for different impurities. The swelling and resistive changes are primarily caused by the absorption of oxygen from the process gas being purified. In addition most of the getter materials have a high capacity for oxygen and moisture. Other impurities, notably, nitrogen and carbonaceous impurities, such as methane and carbon monoxide, not only cause limited swelling of the getter material, but also have much lower affinities for the getter material.
For example, an argon purifier with a consumable bed has extremely high capacity for O2 and H2O, but limited capacity for N2 and CH4. The end of life sensor will trigger the alarm if the purifier is reaching capacity for O2, but not necessarily if it has reached its capacity for N2. Thus, the end of life sensor can indicate that the purifier is performing properly and still be allowing N2 into the purified process gas. This is of particular concern as N2 is usually the highest concentration impurity in the argon supplied to the purifier.
The end result is that the end of life sensor could indicate that the purifier is still performing properly, but the purifier may have broken through for one or more specific impurities such as nitrogen and/or methane. Also these approaches work only for certain classes of getter materials, in the patents described above the transition metal based getter materials. If different materials and purification chemistries are used they may not be amenable to the end of life sensor approach.
A different approach is disclosed in U.S. Pat. No. 5,334,237. It describes the use of a sacrificial model purifier to determine when the main or system purifier is spent. The model purifier made of the same material as the larger system purifier is fed a measured proportional flow of the process gas. If the model purifier is consumed in 6 months, for example, and has 20% the capacity of the system purifier, the system should then last three years. This approach introduces several complications. First, it requires analysis with one or more expensive analyzers and someone skilled to operate them. This loses a major advantage of the end of life sensor approach. Second, it calls for a model purifier of 25%-50% of the capacity of the system purifier and additional process gas flow control and monitoring. For large process gas flows, this could represent a complex system with increased capital equipment costs. Third, after the model purifier has been expended, extrapolation of the lifetime of the system purifier assumes that the inlet impurity challenge remains constant over the lifetime. This may or may not be a true assumption.